There is an existing thread about how to control the TVIS here -> http://www.celicatech.com/forums/showthread.php?t=3497, but there isn't much of an explaination as to why you should use it.. and what the hell is it for anyway? Except for dropping screws into the combustion chamber and breaking all your expensive parts?

The Toyota Variable Induction system was introducted for the 3S-GTE engine for the 4th generation Celica GT4/Alltrac (chassis code: ST165). The purpose of the mechanism is to allow the engine to use a much longer intake runner then would normally be allowable for the operating rpm range of the engine. The longer the intake runner (given that the length per section is of approximately the same internal volume as the rest) the higher the velocity will be at the end of the intake port (above and at the valve) because the vacuum transmitted from the cylinder has more time to act on the air. This is good because a higher velocity will pack more of the air/fuel into the combustion chamber as it has more inertia (velocity * mass), however, above a certain speed, the liquid fuel will fall out of suspension with the faster moving air as the fuel droplets have a much higher inertia (due to the higher mass of the liquid fuel) so they can't make sharp turns (going around the valve head IS a sharp turn), as such the fuel forms droplets and combustion efficiency suffers. The TVIS keeps this from happening.

The TVIS starts with the intake manifold, which utilizes 2 runners per cylinder. The runners sweep down into the actual TVIS mechanism which consists of a throttle plate type assembly where the intake manifold meets the cylinder head. It is arranged such that one of each of the two runners feeding each cylinder is blocked off by default, restraing flow into the cylinder head. Because, at low engine speed, the volume of the single runner is sufficient to feed the engine, there is still plenty of air to feed the cylinder, but because the total area is reduced, velocity will increase in the remaining area. The actual control mechanism of the TVIS is a simple vacuum switching valve, when the ECU registers that certain conditions are present, it will open the TVIS plates, making more total area available and lowering overall velocity. The total area is sufficient for the engine to run to a higher rpm limit without losing fuel suspension.

So don't throw away an excellent system unless you're going to make something better, take it apart, streamline it, clean up the manifold and the head, stake the screws and locktite them.

If you want to improve the manifold, hack off the plenum, make better runner entries and a plenum that empasizes equal distribution, and take the control of the TVIS away from the ECU. The intake manifold I had made for myself has completely new runners and a new plenum, it also has 1 injector on each of the 8 runners (staged injection ftw), but it still retains the TVIS, though you would hardly recognize it. There are ways to calculate the "optimum" opening point, but because of things outside the range of this explaination (exhaust, restrictions into the intake, turbo, etc) that affect the volumetric efficiency of each vehicle. The best "all around" way is to dyno the car twice, once with it open, once with it closed, and open the TVIS where the torque curves cross.